1. Field of the Invention
The present invention relates to an implantable sensor lead for sensing mechanical cardiac activity of a person, to an electronic device, an implantable cardiac stimulator as well as a method for sensing mechanical cardiac activity of a person.
2. Description of the Prior Art
Sensor signals can be used for purposes such as optimization of biventricular synchronization or setting the AV-delay in connection with cardiac stimulation, diagnosis of congestive heart failure, etc.
Leads carrying mechanical sensor elements, like accelerometers, pressure sensing elements, strain gauges or tensiometers, often suffer from disturbances caused by local forces acting on the sensor elements. Thus, such a sensor element placed on e.g. the left side of the heart in a coronary vein is heavily influenced by local forces close to the sensor element. Similar problems occur in connection with the use of so-called CMES-sensors, Cardio Mechanical Sensors, placed in the right ventricle, in coronary sinus, in great cardiac vein or in a coronary vein. Moreover, the placement of the sensor element of a cardiac sensor is seldom freely selectable.
Sensing elements having a certain extension are previously known as well as the use of more than one sensing element spread out over a part of the heart.
In EP 0 473 070 a myocardial tensiometer incorporated within an implantable electrotherapy apparatus to measure the contraction of the heart muscle is described. The tensiometric element consists of piezoelectric material or a variable resistivity material, the mechanical stresses to which the tensiometric element is subjected causing the element to produce a voltage or resistivity variation. The tensiometric element is disposed at a location which is subject to bending when the heart contracts. Thus the tensiometric element may be in the form of a strip disposed on a surface of a patch electrode, or a strip or a tube located at the bend of an implantable J-shaped pacing lead.
U.S. Pat. No. 5,423,883 discloses an implantable myocardial lead. At the distal end of the lead a plurality of appendages are disposed. These appendages are intended to embrace the tissue of the heart and will flex forward and backward to move with the tissue of the heart. At least one sensor element, like a piezoelectric crystal or an accelerometer, is secured to an appendage for detecting the heart wall motion.
In WO 95/03086 implantable leads incorporating accelerometer-based cardiac wall sensors are described. The sensed cardiac wall motion is used to discriminate among potentially malignant cardiac arrhythmias. The cardiac wall motion sensor may be incorporated in a flexible epicardial patch electrode or be incorporated in an endocardial lead.